1. Field of the Invention
The present invention relates to the area of non-destructive film thickness measurement. In particular, the present invention relates to a method and apparatus for improving the resolution and accuracy of x-ray reflectometry measurements of thin films.
2. Discussion of Related Art
Conventional thin film thickness measurement systems often use a technique known as x-ray reflectometry (XRR), which measures the interference patterns created by reflection of x-rays off a thin film. FIG. 1A shows a conventional x-ray reflectometry system 100, as described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997 to Koppel. X-ray reflectometry system 100 comprises an x-ray tube 110, an x-ray reflector 120, a detector 130, and a stage 140. A test sample 142 having a thin film layer 141 is held in place by stage 140 for the measurement process.
To measure the thickness of thin film layer 141, x-ray tube 110 directs a source x-ray beam 150 at x-ray reflector 120. Source x-ray beam 150 actually comprises a bundle of diverging x-rays, including x-rays 151, 152, and 153. X-ray reflector 120 reflects and focuses the diverging x-rays of x-ray beam 150 into a converging x-ray beam 160. Converging x-ray beam 160 includes x-rays 161, 162, and 163, which correspond to x-rays 151, 152, and 153, respectively. Typically, x-ray reflector 120 is a monochromator that ensures that only x-rays of a particular wavelength are included in converging x-ray beam 160.
Converging x-ray beam 160 is then reflected by thin film layer 141 as an output x-ray beam 170 onto detector 130. A detail view of this reflection is shown in FIG. 1B, with reflected x-rays 171, 172, and 173 corresponding to incident x-rays 161, 162, and 163, respectively. The x-rays undergo specular reflection, forcing angles A1, A2, and A3, of x-rays 161, 162, and 163, respectively, to be equal to angles A11, A22, and A33 of x-rays 171, 172, and 173, respectively.
As shown in FIG. 1C, the reflected x-rays are actually formed by reflections at both thin film surface 141a and thin film/substrate interface 142a. Using x-ray 162 as an example, the incident x-ray splits into a primary ray 172a and a secondary ray 172b at thin film layer 141. Primary ray 172a is reflected by thin film surface 141a at an angle A22. Secondary ray 172b is transmitted through thin film layer 141 and is reflected at thin film/substrate interface 142a, eventually exiting thin film surface 141a at angle A22.
Because both rays 172a and 172b exit thin film surface 141a at angle A22, the intensity of x-ray 172 is determined by the amount of constructive or destructive interference between the two rays. The two rays will be in phase if the difference between the optical path length of primary ray 172a and the optical path length of secondary ray 172b is equal to an integer multiple of the wavelength of x-ray 162. (Note that the optical path length of ray 172b includes the distance secondary ray 172b travels within thin film layer 141 multiplied by the index of refraction of thin film layer 141.) If rays 172a and 172b are in phase, the maximum intensity for x-ray 172 is achieved. However, if this optical path length difference is not an integer multiple of the wavelength of x-ray 162, then the two rays will be out of phase, thereby reducing the intensity of x-ray 172.
Note that the actual optical path length of secondary ray 172b within thin film layer 141 is controlled by the incident angle of x-ray 162. Therefore, the intensity of x-ray 172 is ultimately determined by incident angle A2. By simultaneously focusing a beam of x-rays spanning a range of incident angles at the thin film layer, a reflected beam of x-rays having varying intensities can be generated. Those varying intensities can be measured by sensor 130, as indicated in FIG. 1B. For example, reflected x-rays 171, 172, and 173 are shown impinging on a detector plane 130a of detector 130 at points 181, 182, and 183, respectively. Points 181, 182, and 183 typically comprise sensor pixels capable of measuring incident x-ray intensity. The known pixel positions allow detector 130 to correlate the intensities at points 181, 182, and 183 with incident angles A1, A2, and A3, respectively. By performing a similar correlation for all the pixels on detector surface 130a, a reflectivity curve can be derived for thin film layer 140. An example reflectivity curve is shown in FIG. 2. By measuring the fringes in the reflectivity curve, the thickness of thin film layer 140 can be determined, as described in U.S. Pat. No. 5,619,548.
However, accuracy of conventional x-ray reflectometry systems can be severely limited by problems associated with x-ray scattering and spreading at the thin film surface. For example, FIG. 3 shows a detail view of x-ray reflectometry system 100, with incident x-rays 164 and 165 being reflected by thin film layer 141. X-ray 164 has an incident angle A4 and is reflected at an angle A44 as x-ray 174. In accordance with the law of specular reflection, angle A4 is equal to angle A44. X-ray 165 has an incident angle A5, and theoretically would be reflected at an angle A55 as x-ray 175r, where angle A55 is equal to angle A5. Because angle A4 is different from angle A5, x-rays 174 and 175r would ideally impinge on detector surface 130a at points 184 and 185, respectively. However, scattering caused by imperfections in the surface of thin film layer 141 can result in a portion or all of the incident x-rays parallel to x-ray 165 scattering off as x-ray 175s. X-ray 175s leaves the surface of thin film layer 141 at an angle A5s (which is not equal to incident angle A5). If angle A5s happens to be equal to angle A44, both x-rays 175s and 174 will impinge on detector surface 130a at point 184, thereby corrupting the intensity measurements at both points 184 and 185. Scattering is most likely to occur for x-rays having incident angles near the “critical angle” where total external reflection takes place.
The accuracy of conventional x-ray reflectometry systems is further degraded by problems associated with x-ray beam spreading. For example, FIG. 4A depicts the interface between x-ray beam 160 and thin film layer 141 where an illuminated spot B (i.e., the spot formed by the intersection between thin film layer 141 and x-ray beam 160) is formed on thin film surface 141a. Compared to a cross-section A at the most tightly focused portion of x-ray beam 160, illuminated spot B is significantly elongated in the beam direction. FIG. 4B shows cross-section A of x-ray beam 160 overlaid onto illuminated spot B. Conventional x-ray tubes produce an approximately circular x-ray beam, as indicated in FIG. 4B. Accordingly, the height H1 and width W1 of cross-section A are the same (i.e., unitary aspect ratio). In contrast, illuminated spot B is significantly distorted as it spreads across thin film surface 141a, and so has a length L2 and a width W2 at its largest dimensions. In a direction perpendicular to the beam direction and parallel to thin film surface 141a (sometimes referred to as the “neutral axis”), width W2 of illuminated spot B is increased slightly from width W1 of beam cross-section A. However, along the beam direction, height H1 of beam cross-section A is translated into a significantly greater length L2 of illuminated spot B. This disparity in x-ray beam height and illuminated spot length increases as the incident angle of the incoming x-ray beam decreases, and so is particularly problematic for the grazing-angle x-ray beams required in x-ray reflectometry. For example, at an incident angle of 0.5 degree, the length of the illuminated spot is roughly 100 times greater than the diameter of the x-ray beam.
Because of this lengthening of the illuminated spot, the resolution of conventional x-ray reflectometry systems can be degraded in two main ways. First, the increased illuminated spot size increases the chances that irregularities in the surface of the thin film will lead to scattering of the incident x-rays. Second, the larger spot size can allow reflections of x-rays having different incident angles to impinge on the same point on the detector. For example, if an x-ray reflects from the thin film layer at a point farther from the detector surface than an x-ray having a larger angle of incidence, both reflected x-rays could converge at the same pixel on the detector surface, thereby improperly skewing the measured results. This “overlapping” reflection becomes progressively more prevalent as the spreading of the illuminated spot increases, and can ultimately prevent any measurement of the thin film layer thickness. Note that the increase in illuminated spot width does not present a problem since the key reflection and intensity measurements are all along the beam direction.
In addition, it is desirable in semiconductor manufacturing to measure in as small a region as possible. It is especially desirable to measure within the scribe line (i.e., the region between active integrated circuits), which is usually less than 100 μm wide. Furthermore, even within the scribe line, space is very valuable and so it is desirable to limit the length of the measurement spot within the scribe line. The scribe line may contain patterned regions used for other types of tests, and the x-ray reflectivity measurement is best done on an unpatterned region. It is therefore desirable that the measurement be made in as small a region as possible, ideally in a region less than 100 μm×100 μm.
Therefore, it is desirable to reduce the illuminated spot size at a measurement location in an x-ray reflectometry system. However, reducing the x-ray beam diameter in conventional x-ray tubes can be difficult. X-rays are produced by aiming a high-energy electron beam (e-beam) at a metal target. This electron bombardment causes the target atoms to emit x-rays, but also significantly heats the exposed portion of the target. Since the energy level of the electrons in the e-beam must remain at a specific level to cause the target atoms to emit the desired x-rays, reducing the cross-sectional area of the e-beam increases the energy flux at the exposed portion (source spot) of the target. This in turn increases the required rate of heat conduction away from the source spot to prevent overheating. Therefore, the minimum size of the e-beam is constrained by the thermal conductivity of the target material surrounding the perimeter of the source spot.
Accordingly, it is desirable to provide an x-ray reflectometry system that minimizes the effect of beam spreading at a measurement location while maintaining sufficient x-ray flux at the measurement location.